Added explanation for "no changes made" error

Added a documentation page that explains the
"no changes made" error.

Change-Id: I97e9fcad7c1f91eee0479b31106fec0ac1eaaad4
Signed-off-by: Edwin Kempin <edwin.kempin@gmail.com>

Documentation/error-messages.txt

@@ -15,6 +15,7 @@ Error Messages
 * link:error-invalid-changeid.html[invalid Change-Id line format in commit message]
 * link:error-missing-changeid.html[missing Change-Id in commit message]
 * link:error-multiple-changeid-lines.html[multiple Change-Id lines in commit message]
+* link:error-no-changes-made.html[no changes made]
 * link:error-no-common-ancestry.html[no common ancestry]
 * link:error-no-new-changes.html[no new changes]
 * link:error-not-a-gerrit-administrator[Not a Gerrit administrator]

Documentation/error-no-changes-made.txt

@@ -0,0 +1,19 @@
+no changes made
+===============
+
+With this error message Gerrit rejects to push a commit as a new
+patch set for a change, if the pushed commit is identical with the
+current patch set of this change.
+
+A pushed commit is considered to be identical with the current patch
+set if
+- the files in the commit,
+- the commit message,
+- the author of the commit and
+- the parents of the commit
+are all identical.
+
+
+GERRIT
+------
+Part of link:error-messages.html[Gerrit Error Messages]